The present invention relates to rolling bearing well adaptable for the supporting of a fly wheel used for the attitude control of a man-made satellite.
As a conventional technique for controlling an attitude of a man-made satellite is such a manner that a fly wheel is installed on the satellite, an angular rate of rotation of the fly wheel is varied to apply a reaction torque to the satellite. In the controlling of the satellite attitude, a rolling bearing for supporting the fly wheel, which has been used, is a pre-loaded combination angular ball bearing, for example. For the fly-wheel support rolling bearings at the satellite attitude control, the following items are required: good durability and less dust generation under vacuum condition, low torque, little torque variation, long lifetime, and the like. To satisfy those required items, it is required that the lubricant oil is stably supplied to the rolling bearing while keeping the least amount of lubricant oil.
In this type of rolling bearing, for a fixed period of time continued till an external lubricant oil source starts to supply lubricant oil to the rolling bearing, it is lubricated with a slight amount of lubricant oil coated over the raceway surfaces of the inner and outer rings, and also with the lubricant oil impregnated into the retainer.
As described above, the rolling bearing is lubricated with a slight amount of lubricant oil till the external lubricant oil source starts to supply lubricant oil to the rolling bearing. During this period, an undesired phenomenon sometimes occurs: a band mark (band wear) appears on the surface of the ball (rolling element) or the luster of the ball surface is lost. A major cause of the band mark is slip between the balls and the inner and outer raceway surfaces in a state that those are insufficiently lubricated. It is difficult to avoid those damages by merely removing foreign materials or improving washing work.
The word, xe2x80x9cdamagexe2x80x9d, means a damage of the ball to such an extent that the band mark and the losing of the luster, which are formed on the ball surface, can be perceived by the eye, but a degree of wearing is extremely small, and the wear is barely recognized in the surface roughness. However, for a long working, the wear of such a degree will grow under a special circumstance, such as, in the outer space, and consequently the rolling bearing will lose its function.
Accordingly, an object of the present invention is to provide a rolling bearing for a man-made satellite, which is free from the damages, e.g., formation of the band mark on the surfaces of the rolling elements, and the losing of the luster of the surfaces thereof.
Much effort has been made to find the way to prevent the surface damage of the rolling elements in the special conditions of the outer space in a manner that a proper modified layer is formed on the surface of a component part of the rolling bearing while utilizing the basic characteristic of steel as intact. The inventors reached the following technical idea to prevent the surface damage.
The technical idea is implemented in the form of a rolling bearing in use for a man-made satellite. The rolling bearing comprises an inner ring having an inner raceway surface, an outer ring having an outer raceway surface, and a rolling element rotatably disposed between the outer raceway surface and the inner raceway surface, wherein the inner and outer rings, and the rolling element are made of martensitic stainless steel as the parent material, the rolling element includes a nitride surface layer that has a hardness of Hv1200 to 1500.
In particular, in the above-mentioned structure it is advantageous that a surface roughness of the nitride surface layer is 0.1 xcexcmRa or less.
In particular, in the above-mentioned structure it is advantageous that a thickness of the nitride surface layer is 3 xcexcm or greater and 2% DA or less, where 2% DA represents 2% of the diameter of the rolling element.
In particular, in the above-mentioned structure it is advantageous that the martensitic stainless steel consists of AISI 440C, a longitudinal elastic modulus of the AISI 440C is 200 GPa, and a longitudinal elastic modulus of the nitride layer is 240 GPa.
In particular, in the above-mentioned structure it is advantageous that a uniformity thickness of the nitride surface layer is preferably within 5 xcexcm.
Furthermore, in the above-mentioned structure it is advantageous that at least one of said inner and outer raceway surfaces also include said nitride surface layer.
In the above constructed rolling bearing, even if the oil films among the inner and outer rings, and the rolling elements reduce in their thickness with progress of the operation of the rolling bearing, and the surfaces of those are brought into contact with one another, at least the surfaces of the rolling elements are not damaged since the Vickers hardness of those elements are set at 1200 to 1500 (Hv=1200 to 1500). A longitudinal elastic modulus of the nitride surface layer is larger than that of the martensitic stainless steel. Therefore, a contact ellipse is extremely small, and no slip occurs. As a result, there is no chance that a band mark is formed on the rolling elements.
The rolling bearing of the invention is assembled into rotary parts of the man-made satellite. Examples of the rotary parts of the man-made satellite are the fly wheel, the paddle developing mechanism of a solar battery, the antenna developing mechanism, the actuators of the docking mechanism, manipulators of the robot arms, radiation meter, and switches.
Examples of the process of forming a nitride surface layer (having a proper nitrogen concentration and a proper hardness) are liquid nitriding process using a salt bath, gas nitriding process, and ion nitriding process.
Of those processes, the ion nitriding process has a relatively high process temperature. Accordingly, it is difficult to secure a satisfactory hardness of the substrate even if the heat resistance of the parent material is taken into consideration. Sometimes, it is difficult to form a uniform nitride surface layer over the surface of the spherical object. For this reason, it is preferable to use the salt bath nitriding process or the gas nitriding process. Incidentally, 480xc2x0 C. or lower is preferable for the nitriding process temperature.
In particular when the process temperature is high, a compound fragile layer of several xcexcm (pseudo ceramic layer of "xgr" phase or a single xcex5 phase) is often formed like a film on the outermost surface of the nitride layer. To avoid this, the nitriding process temperature is more preferably 460xc2x0 C. or lower. At such a low process temperature, the nitride surface layer is more densely and is free from the formation of bad porous layers.
A great amount of one or more nitrides of those "xgr" phase (Fe2N), xcex5 phase (Fe2-3N), xcex3xe2x80x2 phase (Fe4N), CrN and Cr2N separate out into the martensite, whereby forming the nitride surface layer of the invention. Accordingly, the nitride surface layer is extremely hard and tough. Those properties of the nitride surface layer greatly suppresses the damaging of the rolling element.
Further, since the parent material is martensitic stainless steel, the substrate has a satisfactory hardness of about HRC57 to 59 even under the nitriding process conditions.
Preferably, the rolling elements having the nitride surface layers formed thereon are subsequently subjected to a finishing process, and its surface roughness is 0.1 xcexcmRa or less. By so done, the nitride surface layer of the rolling element will come in contact with its counterpart with less impact.
A method of manufacturing rolling elements will be described hereunder.
To start, stock balls are formed using a wire formed by cold drawing, by cold or cutting process and flushing by the header. The formed stock balls are quenched, and tempered, and if necessary, is also subjected to sub-zero process, whereby it is hardened. Thereafter, those balls are each ground up to a target dimension, i.e., product dimension+marginal dimension, whereby semi-product balls are formed. The marginal dimension includes a marginal dimension necessary for the precise finishing work to gain a target dimension, and further the expansion/compression by the nitriding process.
The quenched stock balls before the nitriding process are greatly large in roundness deviation and diameter difference. Accordingly, a marginal dimension of about several tens 10 xcexcm to several hundreds xcexcm is generally required. Accordingly, if nitride surface layers are formed on the quenched balls, the nitride surface layer is cut nonuniformly, and uneven in thickness. Further, the balance of inner stress of the ball is caused by the nitriding process is lost. As a result, a long time is consumed for the finishing work to gain a required dimension, or a target precision cannot be obtained. Further, this adversely affects the durability of the rolling elements, sometimes.
Those problems are remarkable in the quenched stock balls. When the semi-product balls have unsatisfactory dimensional accuracy, a required dimensional precision cannot be obtained or much time is used for the finishing work. Accordingly, the roundness deviation of the semi-product ball is 3.0 xcexcm or smaller, preferably 1.0 xcexcm or smaller.
The quenched semi-product balls must be tempered. If not tempered, internal residual stress accumulated in the balls will adversely affect the quality of the completed products. For this reason, it is preferable to temper the quenched balls to such an extent as not to lose a required hardness of the substrate. To prevent formation of scratch on the ball surface or to secure a required strength of it, in the process of forming semi-product balls, after a heart treatment, the balls may further be hardened by mechanical hardening process, e.g., barrel or ball peening.
The resultant rolling elements have uniform nitride surface layers being extremely hard, Hv1200 to 1500 and excellent in roughness. Further, the substrate supporting the nitride surface layer is formed to so as to satisfy HRC57 or higher. Accordingly, also when the bearing is operated under large load conditions, the necessary durability is secured.
When the nitride surface layer is too large in thickness, the manufacturing cost is high, and the expansion/compression and deformation of the products are large. Accordingly, finishing cost is also increased. The physical properties of the product is deteriorated. A preferable thickness of the nitride surface layer is 3 xcexcm or larger and 2% Da (2% of the diameter). For this reason, a uniformity thickness of the nitride surface layer is preferably within 5 xcexcm.
Description has been made about the rolling elements of the ball bearing. The same thing is true for the rolling elements (rollers) of other rolling bearings.